feature

A complete acceleration measurement system; monolithic integrated circuit; available over ±35 g, ±50 g, or ±70 g output full scale; fully differential sensor and high resistance circuitry; to EMI/RFI; environmentally friendly packaging; complete Mechanical and electrical self-test; digital command; output ratio supply; sensitive axis on chip plane; high linearity (0.2% of full scale); frequency response down to DC; low noise; low power consumption (1.3 mA); precision Sensitivity tolerance and 0 g offset capability; maximum usable pre-filter holding headroom; 400 Hz, 2-pole Bessel filter; single-supply operation; compatible with tin/lead and lead-free soldering processes; suitable for automotive applications.

application

Vibration monitoring and control; vehicle collision sensing; shock detection.

General Instructions

The ADXL78 is a low-power, complete single-axis accelerometer. Monolithic integrated circuit with signal conditioning voltage output. The Product measures acceleration over a full-scale range of ±35 g, ±50 g, or ±70 g (min). It can also measure dynamic acceleration (vibration) and static acceleration (gravity). ADXL78 is ADI's fourth representative surface micro-mechanical iMEMS 174 ; accelerometer, higher performance and lower cost. Specifically designed for frontal and side impact airbag applications, it also provides a complete cost-effective solution for a variety of other applications. Temperature stable and accurate over the automotive temperature range, the ADXL78 has a self-test feature that fully trains all mechanical and electrical components of the sensor with a digital signal applied to a single pin.

The ADXL78 is available in an 8-terminal ceramic LCC package of 5 mm x 5 mm x 2 mm.

theory of operation

The ADXL78 provides a fully differential sensor structure and circuit path resulting in the industry's highest resistive EMI/RFI effects. The latest generation employs electrical feedback with zero force feedback for improved accuracy and stability. The sensor resonant frequency is significantly higher than the signal bandwidth set by the on-chip filter, avoiding analytical problem bandwidths caused by formants near the signal signal.

Figure 5 is a simplified view element of a differential sensor. Each sensor includes several differential capacitor units. Each battery consists of a fixed plate mounted on the base plate and a movable plate fixed on the frame. Displacement of the frame changes the differential capacitance, measured by the on-chip circuitry.

Complementary 400 kHz square wave drives the mounting plate. Electrical feedback adjusts the amplitude of the square wave so that the AC signal on the moving board is zero. The feedback signal is linearly proportional to the applied acceleration. This unique feedback technology ensures that there is no electrostatic force exerted by the network on the sensor. The differential feedback control signal is also applied to the input of the filter, which is filtered and converted to a single-ended signal.

application

Power decoupling

In most applications, a 0.1µF capacitor C provides sufficient isolation of the accelerometer from noise on the power supply. However, in some cases, especially in the presence of noise at the 400khz internal clock frequency (or any of its harmonics), noise on the power supply can interfere with the output of the ADXL78. If additional decoupling is required, a 50Ω (or less) resistor or ferrite bead can be inserted on the power line. Alternatively, a larger bulk bypass capacitor (in the 1µF to DC 4.7µF range) can be added in parallel with C.

self test

A stationary finger in a forced battery usually remains at the same potential as the active frame. When the self-test digital input is activated, the voltage on the stationary finger on the side of the mobile board in the forced battery changes. This creates an attractive electrostatic force that moves the frame towards those fixed fingers. The entire signal channel is active; therefore, sensor displacement results in a change in V. The ADXL78 self-test feature is a comprehensive method to verify the operation of the accelerometer.

Since the electrostatic force is independent of the polarity of the voltage on the capacitor plates, a positive voltage is applied to one half of the forced cells and a positive voltage to the other half of the forced cells. Activating the self-test will cause a step function force to be applied to the sensor while cancelling the capacitive coupling term. The ADXL78 has improved self-test capabilities, including excellent transient response and high-speed switching capabilities. Arbitrary force waveforms can be used to measure the frequency response of the system by modulating the sensor's self-test input, such as a test signal, or even a crash signal to verify that the algorithm is within the self-test swing.

The ST pin should not be exposed to voltages greater than V+0.3 V. If this cannot be guaranteed due to system design (for example, if there are multiple supply voltages), a low-V clamp diode between ST and V is recommended.

clock frequency supply response

In any clocking system, power supply noise near the clock frequency can have an effect at other frequencies. The internal clock usually controls the sensor excitation and the signal demodulator of the micromachined accelerometer.

If the power supply contains high frequency spikes, it can be demodulated and interpreted as an acceleration signal. The signal appears as the difference between the noise frequency and the demodulator frequency. If the power supply peak is 100 Hz from the demodulator clock, there is an output term at 100 Hz. If the power clock is exactly the same frequency as the accelerometer clock, the term appears as an offset.

If the difference frequency exceeds the signal bandwidth, the filter will attenuate it. However, the power clock and accelerometer clock may vary with time or temperature, which can cause interfering signals to appear in the output filter bandwidth.

ADXL78 solves this problem in two ways. First, the high clock frequency simplifies the task of choosing the power clock frequency so that the difference between it and the accelerometer clock is kept outside the filter bandwidth. Second, the ADXL78 is the only micromachined accelerometer with a fully differential signal path including differential sensors. Differential sensors remove most of the power supply noise before reaching the demodulator. Good high-frequency power supply bypassing, such as ceramic capacitors close to the power pins, can also minimize the amount of interference.

Clock Frequency Supply Response (CFSR) is the ratio of the response at V to the noise on the supply near the accelerometer clock frequency. A CFSR of 3 means that the signal at V is 3 times the amplitude of the excitation signal at V near the accelerometer's internal clock frequency. This is similar to the power response, except that the frequency of the stimulus and response is different. The CFSR of the ADXL78 is 10 times higher than that of a typical single-ended accelerometer system.

signal distortion

Signals from collisions and other events can contain high-amplitude, high-frequency components. These elements contain very little useful information and are reduced by a 2-pole Bessel filter at the output of the accelerometer. However, if the signal saturates at any point, the accelerometer output does not look like a filtered version of the acceleration signal.

The signal may saturate anywhere before the filter. For example, if the resonant frequency of the sensor is lower, the displacement per unit of acceleration is higher. If the applied acceleration is high enough, the sensor may reach the mechanical travel limit. This can be solved by positioning the accelerometer where it cannot see high acceleration values, and by using a higher resonant frequency sensor such as the ADXL78.

Also, in overload conditions between the sensor output and the filter input, the electronics can saturate. Electrical saturation can be minimized by ensuring linear operation of internal circuit nodes at least several times the full-scale acceleration value. The circuit of the ADXL78 is linear about 8 x full scale.

Dimensions

1. All models are on tape and reel and are RoHS compliant parts.

2. Z = RoHS compliant parts.

3, W = meet the requirements of automotive applications.

automotive products

The ADW22279, ADW22280, and ADW22281 models are available in controlled manufacturing to support the quality and reliability requirements of automotive applications. Note that specifications for these models may differ from commercial models; therefore, designers should carefully review the Specifications section of this data sheet. Only the automotive grade products shown are available for automotive applications. For specific vehicle reliability reports for these models, please contact your local Analog Devices account representative.